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The T-score on your bone density report shows how much your bone mass varies or deviates from the bone mass of an average healthy 30 year old adult. Bone density is like any other medical test or measurement. The results for the entire population will fall around an average score (the mean). A T-score is a standard deviation -- a mathematical term that calculates how much a result varies from the mean. The score that you will receive from your bone mineral density (BMD or DXA) test is measured as a standard deviation from the mean. The manufacturers have programmed the DXA machines to use a formula to compute these values.
Osteoporosis was defined by the World Health Organization in 1994 as a T-score that is 2½ standard deviations below the mean or LOWER THAN -2.5.
Some people have low bone density. You may hear this condition called osteopenia. Osteopenia is defined by the World Health Organization (WHO) as 10% to 25% below peak adult bone mass, or a T-score between –1.0 and –2.5 standard deviations below normal. Having a T-score between -1.0 and -2.5 is not "pre-osteoporosis" but it is very important for people with low bone density to develop a good plan to prevent bone loss and osteoporosis.
A Z-score compares your bone density to the average bone density of people your own age and gender. For example, if you are a 60-year-old female, a Z-score compares your bone density to the average bone density of 60-year-old females.
NOTE: Any post menopausal woman should always request her T-score.
A Z-score is helpful in diagnosing secondary osteoporosis and for looking at the bone density of children and young adults who have not reached peak adult bone mass. If you have a very low Z-score (more than 2 standard deviations below other individuals your age), your doctor should consider whether other illnesses or medications may be causing lower than expected bone density.
One standard deviation is equal to a 10-12% difference in bone mass. If you are exactly equal to the peak bone mass of an average 30 year old male or female, you do not deviate at all from the average so your T-score would be 0 standard deviations (SD). If your bones are stronger than the average adult, your bone mass may be +1 or +2 SD indicating that your bones have a mass 10 - 20% above that of the average 30 year old. If your bones are less dense than the average adult, your standard deviation may be -2 or -3 indicating that your bone mass is 20-30% below that of the average 30 year old.
Although you may have low bone density when you have your first test, your doctor cannot tell if you have lost bone density or if you have always had lower bone density due to family or medical history. Your peak adult bone mass may have been below that of the average individual. For example, if you have a T-score of -2.5, it is not appropriate to say that you have lost 25% of your bone density. There are lab tests that can help your doctor determine if you are currently losing bone density.
All forms of bone density testing are accurate and useful. But different tests will answer different questions for you and your doctor.
CT (computerized tomography) used in research
Which test your doctor will suggest depends on what she/he is looking for and why. If you are approaching menopause and considering hormone therapy (HT), for example, your doctor may want to know the bone density of your hip and spine to help you decide about HT. (HT can help reduce bone loss related to the loss of estrogen at menopause.) If you are already taking a treatment for osteoporosis, your doctor will want to monitor your progress with a DXA of your spine taken on the same machine every two years. The peripheral measures (heel, wrist or fingers) are not appropriate for monitoring changes since these bones are not as metobolically active as the hip and spine. If central DXA (hip and spine) is not available you might consider a scan of your wrist or heel as an indicator of your bone health. These tests may be valuable because there is an 85% correlation between test results of your hip, spine and other sites. In other words, the bone density in one part of your skeleton will look very much like the density in your other bones if you are like most people. The older you are, the more this relationship holds true. The lower your bone density, the more likely you are to have a bone break because of osteoporosis. We know that hip fractures can be fatal, and spine fractures can be painful and disfiguring.
Knowing your bone density can help you prevent osteoporosis and fractures.
Although central DXA is the best way to predict hip fracture risk, peripheral bone density tests are valuable screening tools.
The major issue in utilizing peripheral DXA as a screening tool is to differentiate those patients with low bone density from those with normal bone density.
As the peripheral measurement gets higher, the chance of the central measurement being low is less. When peripheral measurement is higher than the average peak bone mass of a 30-year old, the chance of a central measurement being low is very small. Your doctor should decide if you need a central DXA is the peripheral measurement is less than 1 standard deviation below the average peak bone mass of a 30-year old (T-score < -1.0).
American Bone Health recommends that if you have a T-score from a peripheral screening test of < -1.0, you should talk with your doctor about further evaluation, especially if you are a postmenopausal women.
For more information concerning these guidelines, visit the International Society for Clinical Densitometry website.
A central DXA (bone density test) is quick, painless and non-invasive. The bone density test will take about 10 minutes, but allow 30 minutes for your appointment. You will be asked to lie flat on a special table. The bone density machine's mechanical arm will scan your hip and spine (unless otherwise requested by your physician). Your radiation exposure is extremely small, approximately 1/10th of that found in a chest x-ray.
If possible, avoid wearing metal on your body (e.g., snaps, underwire, and zippers). Women who are pregnant, or think they may be, should not have the bone density test - either central or peripheral.
In 1994, the World Health Organization (WHO) reviewed the worldwide data on bone density testing and fracture risk. (WHO Study Group. Assessment of Fracture Risk and its Application to Screening for Postmenopausal Osteoporosis. Geneva, 1994). Prior to this report, many bone density testing centers reported results by comparing the patient to other patients of the same age using a Z-score. The WHO concluded that, based on studies of older women, reporting the relation of the bone mass to the average peak bone mass of a 30-year old would be more appropriate.
The WHO suggested that a BMD result from either the spine or hip that is better than or one standard deviation below the average peak bone mass of a 30-year old indicates normal bone density. (T-score > -1.0 = normal) A T-score that is worse than one standard deviation below the average peak bone mass of a 30-year old, but better than 2.5 standard deviations below is considered low bone mass (sometimes referred to as osteopenia). (T-score between –1 and -2.5). A value worse than or equal to 2.5 standard deviations below (T-score < -2.5) the average peak bone mass of a 30-year old indicates osteoporosis. If the bone density measurement is worse than or equal to 2.5 standard deviations the average peak bone mass of a 30-year old and the patient has a fragility fracture, then the patient is said to have established (or severe) osteoporosis.